Surface plasmon resonance (SPR) is used in the nondestructive study of surfaces, interfaces, and very thin layers, and has recently been found to be particularly adapted for the study of immunologic phenomenon such as antigen-antibody reactions and antigen stimulation of tissue. A surface plasmon is an oscillation of free electrons propagated along the surface of a conductor which is typically in the form of a thin metal film of gold, silver or copper. Transverse-magnetic (TM) polarized energy in an evanescent field excites surface plasmons on the thin metal film. The characteristics of the resonance are directly related to the refractive indices of materials on both sides of the metal film. By including the sample to be measured as a layer on one side of the metal film, changes in the refractive index of the sample can be monitored by measuring changes in the evanescent field to surface plasmon coupling efficiency. Surface plasmons represent the quanta of oscillations of surface charges produced by application of an external electric field to a conducting medium.
The surface selectivity of SPR arises from the enhancement of the optical electric fields at metal surfaces when surface plasmon polaritons (SPPs) are created at the metal/dielectric surface. SPPs are coupled photon-plasmon surface electromagnetic waves that propagate parallel to the metal/dielectric interface. The intensity of the optical electric fields associated with an SPP decays exponentially in distance away from the metal surface, with a typical decay length for an SPP into the dielectric being on the order or 200 nm. SPPs cannot be created on an isolated planar metal surface, but rather require a prism or grating coupling geometry for exciting SPPs. Thus, surface plasmon resonance is achieved by using the evanescent wave which is generated when a p-polarized light beam is totally internally reflected at the boundary of a medium having a high dielectric constant, such as glass. The free electron oscillation is affected by the refractive index of the material adjacent the metal surface which forms the basis of SPR measurements.
In a typical SPR scanning angle experiment, p-polarized light from a laser is directed through a prism onto a metal film on which is disposed a thin sample layer being studied. The prism-sample assembly is mounted to a rotation stage, which allows scanning of the incident angle of the laser beam. As the angle of incidence of the laser beam is varied, surface plasmon resonance is evidenced as a sharp dip in the intensity of the laser beam internally reflected within the prism at a particular angle of incidence. The angle of incidence at which resonance occurs is affected by the refractive index of the thin sample layer disposed on the metal film. The angle of incidence corresponding to resonance is thus a direct measure of the characteristics of the thin sample layer. In the case of immunoassays, the measured angle of incidence corresponding to resonance represents a direct measure of the state of reaction between an antibody and its antigen. This SPR method is limited to use over a narrow angular range, limiting the index of refraction measurement range. In addition, lack of precision in the angular control will affect the reproducibility of the SPR measurement results.
Another SPR technique involves the coupling of a laser source to the sample prism in a focused beam and detecting the reflected light with a charge coupled device (CCD) detector. Other methods take advantage of the wavelength dependence of the SPR. Instead of scanning the angle, these methods scan the wavelength by dispersing reflected light into its constituent wavelengths with a light-dispersing prism or grating, and detecting the resonance with a CCD or linear array detector. Yet another technique makes use of a tunable-diode laser to scan the wavelength over the resonance wavelength region.
The present invention addresses the limitations of and improves upon the prior art by coupling a Fourier transform spectrometer to surface plasmon resonance measurements for the study of thin films as well as surface and interface phenomena. Fourier transform spectrometers are widely used for the analysis of adsorption and reflection characteristics of materials. These spectrometers provide intensity modulated light to sampling media with the use of various types of interferometers. These interferometric spectrometers provide fast data acquisition rates, high signal-to-noise outputs, high wavelength precision and reproducibility, and high ordinate precision. In addition, the wavelength region of operation of these spectrometers has a wide range and can be optimized by appropriate selection of light sources, beamsplitter materials, and detectors. The aforementioned characteristics provide improved measurement capability for SPR over prior surface plasmon resonance measurement approaches.